This may make me sound goofy or something, but I can't really describe what I term as "suspicious behavior" . I just get a feeling sometimes about certain people. Maybe it's body language, how they look at things or people, I don't know.

I spent most of my adult life in emergency services as a paramedic, FF, 911 dispatcher, etc. and lived with cops in the family all my life. One thing my dad taught me was to pay attention when you get that sneaking feeling that something ain't right. Over the years, that feeling has kept me from getting struck by unstable patients and/or their friends and family, ducking at the right time before getting shot at, and once got me out of a burning building before the roof collapsed 2 seconds after I crawled out.

With that said, I've never witnessed any real threatening suspicious behavior in or around airports or flights. I've seen tons of WEIRD behavior, but that is just human nature. Some people are just a little different

Safe travels
Tim